Recording with donor transfer of magnetic toner

ABSTRACT

A method and apparatus for selectively transferring magnetic toner from a reservoir to the imaged areas of a copy web. The reservoir is used to develop a donor web in which a multiplicity of microfields have been recorded. The donor web is subsequently passed into non-contacting proximity to a copy web having a latent magnetic image thereon. The toner is selectively attracted to the stronger magnetic forces in the imaged areas of the copy web and remains on the donor web in the non-imaged areas. Another aspect of the invention provides for the neutralization of the donor web microfields prior to the toner transfer to enhance the development process.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention generally pertains to process and apparatus for recordinginformation on a copy sheet by magnetic imaging procedures and is moreparticularly directed to developing a magnetic latent image with amagnetic toner.

2. Prior Art

The processes for providing latent images on a substrate or surface andthen decorating them by a fine pigmented particulate (usually called atoner) to produce a visual image or one that is transferable to a copysheet are well known in the art.

Generally, in the past, a number of development systems have been usedto tone either an electrostatic latent image (zones having electricpotential differences between image and non-image areas) or a magneticlatent image (zones having magnetic potential differences between imageand non-image areas).

Normally, electrostatic and magnetic toners are not compatible.Electrostatic toners typically do not exhibit marked attraction tomagnetic field forces because they are not ferromagnetic while magnetictoners are usually heavy and fairly conductive and are therefore notfavored electrostatic charge carriers.

However, some ferromagnetic materials have been used in electrostaticdevelopment systems as carriers. These ferromagnetic carrier particleswhich are relatively large exhibit triboelectric attractions for smallertoner particles and are useful in transferring the toner to anelectrostatic image. The toner particles are separated from the carrierby the stronger electrostatic forces on the latent images than thetriboelectric forces between carrier and toner.

One example of a cascade development system employing ferromagneticcarrier to transfer a toner to an electrostatic latent image is U.S.Pat. No. 3,545,968 issued to Sato.

Another example of a development apparatus using a ferromagnetic carrieris U.S. Pat. No. 3,437,074 issued to Hagopian et.al. This referencedescribes a "magnetic brush" development system where the ferromagneticcarrier particles are formed into streamers or bristles and form a brushlike mass.

A donor belt utilizing ferromagnetic carrier for toner tranfer in anelectrostatic apparatus is disclosed in a U.S. Pat. No. 3,741,790 issuedto Wu.

Such development systems rely on the electrostatic forces generated bypotential differences in image areas to be stronger than the forcesholding the toner particles to the carriers. The electrostatic forcesgenerated by an electrostatic latent image are in fact much strongerthan those which can be produced from a magnetic latent image and thusother methods had to be initiated to tone these magnetically.

This has led to the development of using ferromagnetic particles in someform that are not just carriers but actual toners for developing amagnetic latent image. As in the electrostatic development area, therehave been a plurality of methods proposed for the decoration of latentmagnetic images by magnetic toner.

A cascade development system for magnetic images is illustrated in U.S.Pat. No. 3,250,636 issued to Wilferth. In this reference, magneticparticulate is poured or flooded over a surface containing a magneticlatent image. The toner adheres to the image areas and excess tonerflows by gravity from the surface into a reservoir.

Another magnetic toner development system, in which the toner is causedto impinge on a magnetic latent image by flicking the toner from thebristle ends of a wire brush, is illustrated in U.S. Pat. No. 3,825,936issued to Ott et.al.

Immersion techniques are also known in the art where a tape has arecorded image thereon immersed in a reservoir within a volatile fluidmedium. Upon circulation of the fluid medium around the image, toner isattracted to magnetized areas of the image. An example disclosing such atechnique is found in U.S. Pat. No. 3,740,265 issued to Springer.

All the aforementioned latent magnetic imaging development apparatushave the problem of contacting toner not only within imaged areas butalso within non-imaged areas and thereby producing substantialbackground. (Toner adhered to non-image areas.)

This is detrimental to a magnetic imaging process as the forces holdingthe magnetizable particles to the image areas are not as great as thosefound in electrostatic system and hence background is more difficult toclean from an area after the toner deposition thereon.

A non-contact magnetic imaging system is illustrated in U.S. Pat. No.3,849,161 issued to Klaenhammer. The system provides alternatemagnetizations for image areas in relation to non-image areas. However,such a system is devoid of a process to produce the resolution needed bymodern imaging applications in the commercial sector.

Therefore, it would be advantageous to have a clean development systemthat would also be capable of uniform image development and highresolution.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the invention to provide an improvedprocess and apparatus for toning or decorating a magnetic latent imagewith ferromagnetic particulate.

It is another object of the invention to provide a contactless toningsystem where only the image areas have particulate adhered thereto.

It is still another object of this invention to reduce background in thedevelopment of a latent magnetic image for a magnetic imaging systemwith high resolution capabilities.

It is a further object of the invention to provide a uniform supply oftoner to a latent magnetic image to produce even image development.

These objects and others are accomplished in accordance with theinvention by providing a donor surface which is magnetizable in analternating pattern of magnetizations of a specified spatial wavelength.The donor surface is toned and then, with magnetic particulate adheringto the microfield pattern, transported into non-contacting proximitywith a magnetizable copy surface. The copy surface has a latent magneticimage recorded thereon which has a stronger magnetic force than that ofthe doner surface. The toner will be transferred by the differential inmagnetic forces due to the fields produced between the copy and donorsurfaces. Only the image areas of the copy surface will be toned asthere are no magnetic force gradients in the non-image areas.

The toner transfer effect is magnified if the copy surface has a highercoercivity than the donor surface according to one aspect of theinvention. Another feature of the invention provides for the enhancementof the transfer process by the erasure or neutralization of the donersurface microfields prior to the toner transfer with a remagnetizationof the donor surface subsequent to transfer.

BRIEF DESCIPTION OF THE DRAWINGS

These and other objects, features and aspects of the invention willbecome clearer and more fully apparent from the following detaileddescription when read in conjunction with the accompanying drawings,wherein:

FIG. 1 is a schematic system diagram of a magnetic imaging apparatusemploying a magnetic doner development process and apparatus inaccordance with the present invention;

FIGS. 2A and B are representative pictorials of sections of the copy weband donor web of the apparatus of FIG. 1 illustrating image domainmagnetizations and pre-recorded microfields, respectively; and

FIGS. 3A, B and C are alternative embodiments of the apparatus andmethod for preforming a toner transfer from the donor web to the copyweb of the imaging system illustrated in FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference now to FIG. 1 there is shown a magnetic imaging systemincorporating the present invention. The magnetic imaging systemincludes a recording station 10 which produces a magnetic latent imageon a copy web 2 in some manner.

There are a number of methods known in the art for accomplishing thisprocess. Some examples are direct recording with a magnetic recordinghead, theremoremanent or anhysteretic copying from a recorded mastertape, Curie point writing or erasure with masks or a laser, etc. Themagnetic latent image that is formed by one of the above-describedprocesses will be an alternating pattern of magnetizations in imagewiseconfiguration. The non-imaged areas are not polarized in anymagnetization direction and the magnetic material in them will produceno magnetic field gradients. The magnetic forces from the fringingfields of the magnetization pattern of the image are thus substantiallythe only attractive forces on the copy web surface.

The copy web 2 is generally a magnetic tape with a magnetizable surfacearea that has a coercivity and paramagnetic state that allow it to bemagnetized in a magnetic image configuration as mentioned before.

Preferred choices for the copy web 2 would be CrO₂ tape sold under thetradename Crolyn by the DuPont corporation (Br = 1600 Gauss, Hc = 5400e,squareness = 0.9) or γFe₂ O₃ tape sold as 3M777 by the 3M Corporation(Br = 1400 Gauss, Hc - 312 Oe, Squareness - 0.8).

The copy web 2 is entrained about copy web rollers 4, 6 and 8 in anendless belt fashion where at least one of the copy web rollers 4, 6 and8 may be driven by conventional motors or other means (not illustrated).Once the copy web has a latent magnetic image produced thereon the imageis transferred by the rotation of the rollers into a toner transfer area12 where it is decorated with a ferromagnetic particulate toner 24 froma reservoir 26. The toners that may be used in the practice of theinvention are ones loaded with soft magnetic material or those loadedwith unpoled hard magnetic materials. Toners such as these are describedin U.S. Pat. Nos. 3,639,245; 2,932,278; 3,052,564 and 3,250,636 thedisclosure of which is herein incorporated by reference.

The magnetic toner 24 is transported to the toner transfer area 12 bymeans of a doner web 16 entrained about three doner web rollers 18, 20and 22. The doner web 16 forms an endless belt around the doner webrollers 16, 18 and 20. At least one of the doner web rollers drives thedoner web 16 and is able to transport the magnetic toner into the tonertransfer area 12.

The doner web 16 is generally a magnetic tape with a magnetizablesurface area that has a coercivity and paramagnetic state that allow itto be magnetized in a microfield pattern of alternating magnetization.Preferred choices are those that have been mentioned above for the copyweb 2 and include Crolyn and 3M777. Additionally, one could use a γFe₂O₃ tape sold as 3M206 by the 3M Corporation (Br = 1303 Gauss, Hc =3320e, squareness = 0.8). This tape has a thicker magnetizable layer(14μ) than the others mentioned and can be used when longer doner webwavelengths are chosen.

The transportation of toner takes place from the reservoir 26 to thetransfer area 12 because the doner web has a magnetizable surfacecontaining a magnetic pattern which attracts the toner while it isdriven over the doner web roller 22.

The toner transfer mechanism for the donor web 16 illustrated in FIG. 1is an immersion technique where the doner web is pulled into a pile orbath of magnetic toner 12 on one side of donor web roller 22 and out ofthe toner on the other. This type of toner development is acceptable forthe doner web 16 as what is needed is a uniform coverage of the entiredoner web.

Other techniques for toning magnetic patterns, such as those discussedin the prior art sections, could be used for this step of the process asthere is no background problem at this point in the process and thenecessity is to transport as much toner in as uniform fashion aspossible.

The magnetization pattern of the doner web 16 causes the tonerparticulate 24 to adhere thereto and to remain on the web until it istransported into the transfer area 12. The magnetization pattern of thedoner web is similar to that of the imaged areas of the copy web andcomprises alternating magnetizations of a specific spatial wavelengthand frequency. However, the doner web magnetizations are of a differentwavelength to produce a weaker magnetic force than that of the copy web2.

In a preferred form the alternations are formed widthwise across thedoner web and parallel to the magnetization patterns in the copy web,although this is not a necessity for the operability of the invention.It should be apparent that the alternating magnetic microfields in thedoner web may, as was explained previously in relation to the copy webpattern, be formed in numerous ways.

The numerous microfields in the doner web provide fringing fields forthe toner 24 to adhere to and produce a substantially uniform tonercoverage on the doner web 16. This is an important aspect of theinvention as the doner web 16 constantly transports an endless supply oftoner in even quantities into the toner transfer area 12. This allowsuniform development of a latent magnetic image without under toning orover toning the copy web 2.

Once in the transfer area 12, the stronger magnetic forces of the copyweb 2 transfers the toner particulate in imagewise configuration ontothe copy web 2 while excess particulate remains on the doner web 16, innon-imagewise configuration. As erase head 14 is used to enhance thistransfer process by demagnetizing the doner web 16 as it passes into thetransfer area 12 before the toner transfer takes place. Alternatively,heating means 40 can be used to demagnetize doner web 16 by raising thetemperature of web 16 above its Curie point. When the erase head 14 isused, a rewrite head 28 is provided to remagnetize the doner web 16before it enters the toner reservoir 26.

As the latent magnetic image having the toner 24 adhered thereto movesout of the transfer area 12 it comes into contact with a copy sheet 23which is held against the copy web 2 by a pair of pressure rollers 25and 27. The magnetic toner transfers in imagewise configuration from thecopy web 2 to the copy sheet 23 under the influence of this pressure.The copy sheet is moved from the supply reel 31 to a takeup reel 29 in acontinuous fashion to provide either multiple copies or individualimages as the copy web 2 continues its endless path.

The copy sheet 23 may then have the toner 24 fixed to this surface insome manner, many of which are known in the art. Subsequently, the copyweb 2 enters the recording station 10 once again where it can be erasedand rewritten with another magnetic image or pass through to pick up orrenew the toner supply from the toner transfer area 12 and continue onto the copy sheet 23 again for multiple copy capabilities.

While the imaging processes and apparatus illustrated in FIG. 1 arepreferred for practicing the invention numerous adaptations areavailable. For example, both the copy web 2 and the doner web 16 couldbe cylindrical drums with a magnetic surface or a non-magnetic drum withan overlayer of magnetizable tape. Combinations are possible where thecopy web 2 can be a drum and a donor web is used or where a copy web 2is provided, a donor drum replaces the donor web for the transfer. Therequired structure is that a magnetizable surface capable of holding thelatent magnetic image pattern and a magnetizable surface capable ofholding the donor pattern be provided.

A portion of copy web 2 is illustrated in FIG. 2A where an image area 30is shown with a magnetic recording pattern 32. The recording pattern 32is actually a series of alternating magnetizations of a certain spatialwavelength and frequency. Where the magnetization sections oppose,fringing fields will be developed to attract the magnetic particulate 24thereto. The spatial frequency for the image magnetization reversalswould be typically on the order of 25 - 100μ. Likewise, in FIG. 2B thereis shown a portion of the donor web 16 having a second magnetizationpattern 34 recorded thereon. It is seen that the magnetization pattern34 is comprised similarly of alternating domains having a certainspatial wavelength and frequency to produce fringing fields for theattraction of toner where the domains are in opposite. It should beparticularly noted that the donor web 16 magnetization pattern 34 is ofa different wavelength than the magnetization pattern 32 of the copy web2. Also it is preferred that the copy web 2 have a different coercivityand Curie Point temperature than the donor web 16 as will be more fullydiscussed herein below.

With reference now to FIG. 3 where is shown an enlargement of the tonertransfer area 12 including the copy web 2 and the donor web 16. Thealternations in the magnetization domains of the pattern 32 are shown asthe image area 30 and illustrate the field lines producing the fringingfields and the magnetic force lines that will cause the toner transfer.It is seen that the spatial wavelength of the image pattern 32 isdifferent than that of magnetization pattern 34 to represent the greaterattraction power. The field on the copy web is constructed to produce adifferential in force to accelerate the particles adhering to the donorweb onto the copy web in the image areas. Preferably, the copy web 2 asmentioned before has a greater coercivity than the donor web 16 toinsure this effect.

In the uniform development of the copy web 2, the spacing separating thecopy web 2 and the donor web 16 are important to the transfer process.For different spacings, the wavelength of the donor web will change asit will for different wavelengths of the image recorded on copy web 2.Table 1 is illustrative of the donor wavelength that must be recorded onthe donor web 16 in relation to the copy web 2 spacing and imagewavelength.

                  TABLE 1                                                         ______________________________________                                        IW    DW d = 13μ                                                                              DW d = 18μ                                                                              DW d = 23μ                                 ______________________________________                                        25μ                                                                              ≧162μ                                                         50μ                                                                              ≧58μ                                                                             ≧142μ                                                                            ≧880μ                               70μ                                                                              ≧55μ                                                                             ≧108μ                                                                            ≧241μ                               95μ                                                                              ≧57μ                                                                             ≧102μ                                                                            ≧188μ                               ______________________________________                                         d = spacing between the copy web 2 and the donor web 16 at the point of       transfer.                                                                     IW = the image wavelength.                                                    DW = the donor wavelength.                                               

It is understood that because there are no magnetization domainsrecorded in the non-imaged areas of the copy tape there are no forcefields to accelerate the particles to the non-imaged areas. Thus, thisnon-contacting toning system solves many problems found in the art. Alow background is maintained, therefore, by not depositing toner intothese areas and consequently a much simpler clean up process than usualis permissible for the present system.

Also, the back of the copy tape 2 remains untoned so cleaning there isalso unnecessary. Further, since the recorded pattern is weaker thanthat on the copy web 26 an amount of toner 24 corresponding to fulldevelopment of the magnetic latent image may be loaded on it andtransferred nearly uniformly onto the copy web 2.

For further enhancing the toner transfer to the copy web 2, a transferhead 14 may be used to erase the donor web microfields or to provide aneutralizing field opposite to the microfields recorded therein. Thetransfer head 14 then neutralizes the forces holding the toner 24 to thedonor web 16 just prior to the transfer to the copy web 2. This assuresthere will be a larger net force produced by the gradient of themagnetization patterns 32 recorded on the copy web 2. The largercoercive force of the copy web 2 prevents erasure of the latent magneticimage during this process as only a magnetic field less than or equal tothe coercivity of the donor web 16 is required by neutralization.

It should be realized that the erasures of the microfields in the donerweb could also be accomplished by heating the donor web above its Curiepoint temperature thereby erasing the magnetization patterns 34 as doesthe transfer head 14. In such a manner though the Curie temperature ofthe copy web 2 should be above the Curie temperature of the donor web toprevent erasure of the imaged area. In both such cases a re-recording orwrite head 28 is positioned in proximity to the tape to allow for there-recording of the microfields on the magnetizable surface of the donorweb 16 in the aforementioned pattern.

Turning now to FIG. 3B another embodiment of the present invention isillustrated where the copy web 2 and the donor web 16 perform theirferromagnetic transfer in the presence of a coil 36. The coil again actsto selectively erase or neutralize the microfields of the donor web 16while not effecting the field gradients of the copy web 2. It isunderstood that either an a.c. or a d.c. current may be used to providea magnetization in the correct direction to perform this neutralization.If an a.c. field is used, the reversal of the fields may be helpful inthat they to some extent strobe the particles in the transfer back andforth between the two surfaces.

Another alternative embodiment of the imaging transfer system isillustrated in FIG. 3C where the copy drum 41 and the donor web 16 arebrought into proximity in the transfer area 12. In this embodiment it isseen that the copy drum 41 is made of the non-magnetizable surfacematerial 43 and that a magnetic or magnetizable 42 material is setthereon in a relief configuration forming an image 38. The image 38 inrelief has been recorded with the first spatial wavelength of the imagemagnetization pattern and therefore will perform a similar transfer whenbrought into proximity with the toner laden microfields of the donor web16. The toner 24 transfers into the field gradients formed by themagnetic domain opposites of the relief image 38. Again the effect canbe enhanced by the transfer head 14 and the donor web 16 may bere-recorded by the rewrite head 28.

While the invention has been described in detail in relation to a numberof preferred embodiments those skilled in the art will understand thatother changes in form and detail may be made therein without departingfrom the spirit and scope of the invention wherein all such changesobvious to one skilled in the art are encompassed in the followingclaims.

What is claimed is:
 1. A method for developing a latent magnetic imageon a magnetizable copy surface comprising the steps of:forming amagnetic latent image on said magnetizable copy surface by alternatingpatterns of magnetization of a first spatial wavelength; providing amagnetizable donor surface; uniformly magnetizing said donor surface inalternating patterns of magnetization of a second spatial wavelength,whereby said first magnetization pattern produces a magnetic fieldstronger than said second magnetization pattern; attracting magnetictoner to the alternating magnetization pattern of said donor surface;transporting said donor surface substantially uniformly laden with saidmagnetic toner into non-conducting proximity with said copy surface,whereby said toner is transferred from said donor surface to said imagearea of the copy surface under the influence of the stronger magneticfield of the image area.
 2. The method as defined in claim 1 comprisingthe additional steps of:demagnetizing said donor surface while in theproximity of the copy surface to enhance the toner transfer process, andremagnetizing said donor web with the second spatial wavelengthsubsequent to the toner transfer.
 3. In a magnetic imaging systemincluding a station for forming a latent magnetic image as alternatingpatterns of magnetizations of a first spatial wavelength in amagnetizable copy surface, a toner station for decorating said magneticimage with a ferromagnetic particulate, and a transfer station fortransferring said decorated image to a copy sheet; said magnetic imagingsystem being characterized by an improved toner station comprising;amagnetizable donor surface; means for uniformly magnetizing said donorsurface with an alternating pattern of magnetization of a secondwavelength, a reservoir of ferromagnetic particulate for decorating saidlatent magnetic image; means for transferring said particulate to saiddonor surface whereby said particulate is adhered to substantially saidentire donor surface by the second alternating pattern ofmagnetizations; means for transporting said particulate laden donorsurface into non-contacting proximity to said copy surface containingthe latent magnetic image, whereby the particulate transfers to theimage areas of the copy surface and remains adhered to the donor surfacein non-image areas.
 4. A magnetic imaging system as defined in claim 3which further comprises;means for erasing said donor surfacemagnetization pattern prior to the particulate transfer; and a rewritingmeans for remagnizaging said donor surface subsequent to saidparticulate transfer.
 5. A magnetic imaging system as defined in claim 4wherein the coercivity of the copy surface is greater than that of thedonor surface.
 6. A magnetic imaging system as defined in claim 5wherein said erasing means includes an erase head driven by a highfrequency current.
 7. A magnetic imaging system as defined in claim 6wherein said rewriting means includes a write head driven by a highfrequency current.
 8. A magnetic imaging system as defined in claim 5wherein said erasing means includes:means for heating said donor surfacebeyond its Curie temperature while not under the influence of externalmagnetic fields.
 9. A magnetic imaging system as defined in claim 8wherein said rewriting means includes:means for heating said donorsurface beyond its Curie tempeature, and means for cooling said donorsurface while under the influence of an external magnetic field havingsaid second spatial wavelength.
 10. A magnetic imaging system as definedin claim 5 wherein said erasing means are a coil wound such that amagnetic field produced by a high frequency current driven therethroughwill erase said donor surface.
 11. A magnetic imaging system as definedin claim 3 wherein said means for transferring said particulate to thedonor surface includes:means for suspending the particulate in avolatile liquid medium; and means for immersing said donor surface intosaid suspension, whereby the magnetization pattern of donor surfacecauses particulate to adhere thereto.
 12. A magnetic imaging system asdefined in claim 3 wherein said copy surface is an endless tapeentrained about means for tape transporting.
 13. A magnetic imagingsystem as defined in claim 12 wherein said donor surface is an endlesstape entrained about means for tape transporting.
 14. A magnetic imagingsystem as defined in claim 3 wherein said copy surface is a drum havinga magnetizable surface and means for the rotation of said drum surface.15. A magnetic imaging system as defined in claim 14 wherein said donorsurface is a drum having a magnetizable surface and means for rotatingsaid drum surface.
 16. A magnetic imaging system as defined in claim 3wherein said copy surface is an imagewise configuration in relief on anon-magnetizable surface.
 17. A magnetic imaging system as defined inclaim 16 wherein said non-magnetizable surface is a drum including meansto rotate the drum surface.
 18. A magnetic imaging surface as defined inclaim 16 wherein said non-magnetic surface is an endless web entrainedabout means for web transporting.
 19. A magnetic imaging system asdefined in claim 3 wherin said means for transferring said particulateto the donor surface includes;means for conveying the particulate fromthe reservoir; means co-operating with said conveying means forcascading said particulate over said donor surface containing saidalternating magnetization patterns of the second spatial wavelength,said cascading means so positioned that excess particulate will fallback into the reservoir by gravitational forces.
 20. A toner system fordecorating a magnetic latent image of a surface comprising:amagnetizable donor surface having a magnetic field wherein the force ofsaid magnetic latent image is stronger than that of said donor field;means for uniformly toning substantially all of said donor surface witha transferable magnetic toner particulate; means for transporting saiddonor surface into non-contacting proximity of said magnetic latentimage surface whereby said stronger image field will cause the tonerparticulate to transfer.
 21. A magnetic latent image toner system asdefined in claim 20 wherein:said magnetic latent image field is producedby alternating patterns of magnetization having a first spatialwavelength; said donor magnetic field is produced by alternatingpatterns of magnetization having a second spatial wavelength.
 22. Amagnetic latent image toner system as defined in claim 21 wherein thefirst spatial wavelength is greater than the second spatial wavelength.23. A magnetic latent image toner system as defined in claim 22 whereinthe first spatial wavelength is less than the second spatial wavelength.24. A magnetic latent image toner system as defined in claim 21 whereinthe coercivity of the latent magnetic image surface is greater than thecoercivity of the donor surface.
 25. A magnetic latent image tonersystem as defined in claim 24 wherein said latent magnetic image surfacecoercivity is in the range of 200-600 Oersteds.
 26. A magnetic latentimage toner system as defined in claim 25 wherein said donor surfacecoercivity is in the range of 200-600 Oersteds.